1. Field of the Invention
This invention relates generally to temperature compensated crystal oscillators utilizing piezoelectric resonators and more particularly to the alteration of the frequency-temperature characteristic of a piezoelectric resonator of a temperature compensated crystal oscillator.
2. Background of the Invention
As is well known, the frequency-temperature (f-T) behavior of a piezoelectric resonator depends upon the elastic, piezoelectric and dielectric properties of the piezoelectric material as well as the temperature coefficients (TC) of these material quantities. The influence of these material quantities relative to one another depends upon where on the reactance vs. frequency curve the resonator is operated. It has been further shown that the temperature coefficient at resonance differs from that at antiresonance and, as a result, resonators are normally operated between these frequencies by placing a load capacitor in series with the crystal and coupling the series combination of capacitor and crystal resonator into an oscillator. The effect of the capacitor is to adjust the resonator's operating or load frequency until the reactance of the combination is approximately zero so that the crystal resonator is forced to operate in an inductive region with its reactance equal in magnitude to that of the series capacitor. Furthermore, the effect of operation between resonance and antiresonance has been described by Arthur Ballato, the present inventor, in the following publications which are meant to be included herein by reference: "Apparent Orientation Shifts of Mass-Loaded Plate Vibrators", A. Ballato, Proceedings of IEEE, Volume 64, September, 1976, pp. 1449-1450; Temperature Compensated Crystal Oscillator (TCXO) Design Aids: Frequency-Temperature Resonator Characteristics as Shifted by Series Capacitors, A. Ballato, Technical Report, ECOM-4498, U.S. Army Electronics Command, Fort Monmouth, N.J. May, 1977, 59 pp; and "Frequency-Temperature-Load Capacitance Behavior of Resonators for TCXO Application", IEEE Transactions on Sonics and Ultrasonics, A. Ballato, Volume SU-25, Number 4, July, 1978, pp. 185-191.
The foregoing pertains not only to bulk wave resonators, but surface wave and shallow bulk wave resonators as well. Furthermore, it is true for any piezoelectric material e.g. quartz, lithium niobate, lithium tantalate, berlinite, barium titanate etc. Only the size of the effect will vary.
With respect to a series load capacitor, however, two basic limitations are present, namely the resonance-antiresonance region is normally very narrow since adjustment of the value of the load capacitor from infinity (short circuit) to zero (open circuit) only varies the frequency between these limits, and the range of the temperature coefficient adjustment, as opposed to frequency adjustment, is also very small.
Accordingly, it is an object of the present invention to provide an improvement in temperature compensated crystal oscillators.
It is a further object of the present invention to provide an improvement in the frequency-temperature adjustment region of a piezoelectric resonator.
And it is yet another object of the invention to provide a range of frequency-temperature adjustment outside of the resonance-antiresonance region of a crystal resonator incorporated in a temperature compensated crystal oscillator.